1. Field of the Invention
This invention deals with the formation of low resistance self aligned silicide contacts to silicon devices and more particularly to the formation of self aligned silicide contacts forming the V.sub.ss plate of adjacent pull down transistors in static random access memory cells.
2. Description of the Related Art
There is often a need for low resistance contacts to areas of silicon or polysilicon in devices formed in silicon integrated circuit substrates. Metal silicides have been widely used to form low resistance contacts and self-aligned silicide contacts provide an additional advantage.
Self aligned silicide contacts, or salicide contacts, are described in "SILICON PROCESSING FOR THE VLSI ERA", Volume 2, by S. Wolf, Lattice Press, Sunset Beach, Calif. 1990, pages 144-149.
Self aligned silicide contacts are also described in "VLSI TECHNOLOGY", Second Edition, by S. M. Sze, McGraw-Hill, New York, N.Y. 1988, pages 397-400 and 479-483.
In the methods described by Wolf and by Sze contact openings are formed in an oxide layer exposing silicon device regions or polysilicon electrode regions. The metal used to form the silicide is deposited on the substrate covering the exposed areas. The wafer is then heated causing the silicide reaction to take place in those regions where the metal is in contact with the silicon. The unreacted metal is then selectively etched and the contact is formed.
One of the problems encountered in conventional methods of forming self aligned silicide contacts is the formation of metal silicide stringers in regions where the metal is to be removed. Overetching is often required to remove the stringers.
The method of this invention forms self aligned silicide contacts using a method which avoids the formation of stringers.